Various commercial leak detectors are available for detecting gaseous leaks into the ambient atmosphere. As an example a leak detector for detecting refrigerant gas disclosed in U.S. Pat. No. Re 32,552 to Liebermann et al (1987) is incorporated herein for reference. Leak detectors are typically provided with a hand-held probe in order to search for leaks in pipe joints, crevices, and other localized regions. The usual procedure for finding a leak involves two phases: "search" and "verification". In the search phase the probe is scanned or wiped over suspected regions; the probe is moved with reasonably constant motion in search of an indication. During verification the motion is substantially stopped with an attempt to hold the probe directly on the leak for verification and pinpointing its location. Some leak detectors provide an indication of leak size during verification.
The type of gas sensor employed depends on the gas to be detected. As an example some employ the corona apparatus for detecting halogen gas leaks in refrigerant systems. When 1500 to 2000 volts are applied to the fine wire a corona discharge current occurs between the wire and an outside shell which diminishes when trace amounts of halogen gas enters the shell. Another type of gaseous leak detector employs a heated metal oxide sensor whose resistance changes when detecting natural gas. Still another refrigerant leak detector utilizes a heated platinum electrode to form a positive ion current; the ion current between the hot electrode and an outside shell increases in the presence of traces of halogen gas.
In these leak detectors electrical changes in the sensor caused by the presence of the leaking gas in the ambient atmosphere are monitored by circuitry to provide a signal of a leak. Typically, a visual and/or acoustical alarm is incorporated which is actuated by the electrical signal from the sensor.
These leak detectors have two noteworthy features in common: 1) They are subject to false signals or noise arising from local environmental variations. For example the corona sensor for refrigerant will respond to local humidity variations; the heated diode refrigerant sensor is sensitive to smoke; the metal oxide natural gas sensor will detect local thermal variations. 2) The alarm circuitry of each of these leak detectors is substantially direct coupled to the sensor to provide a constant alarm signal when the probe is held steady on a leak and the sensor yields a constant signal. This provision is required to properly use the verification procedure.
By using direct coupled circuitry solely, without filtering, present leak detectors have failed to exploit their maximum detection capabilities. Although filtering or signal processing in the detection circuitry can improve detection by selectively enhancing the signal and by diminishing the effect of environmental noise sources, none of these leak detectors incorporates such circuitry.